The present invention relates to a method of, and apparatus for, measuring the swept frequency transmission response of a communications network, as, for example, an analog cable network, and more particularly, to one which measures on a continuous basis in the presence of normal signal activity without interfering in any substantial manner with the reception of those signals.
Until recently, the usual method of making swept frequency response measurements on a transmission network such as, for example, an analog cable system, was to connect a sweep frequency oscillator directly to the cable system. A detector was placed at some other location with the portion of the cable system under test between the oscillator and the detector. The difference between the output power of the sweep frequency oscillator and the input power to the detector represents the transmission response. The chief disadvantage of this type of measurement is that the portion of the cable under test must be disconnected from the system during the test. Therefore, transmission of operational or normal signals ceases during this period.
Another technique for making swept frequency response measurements on cable systems employs a "summation sweep" technique intended for CATV systems. This technique is similar to the above described system with the exception that normal signals are present when the sweep occurs. In order to distinguish the test signal from normal signals, it is necessary to operate the test signal at a high level, usually 20 dB above the normal signals. In order to minimize the interfering effect of the test signal when it sweeps past the television video carriers, the sweep is accomplished in a very short time interval. Nevertheless, fast sweep speed notwithstanding, a noticeable interference is observed on the television screen the instant the test signal sweeps past the video carrier.